Via wave guide with curved light concentrator for image sensing devices

ABSTRACT

A CMOS image sensor (CIS) device includes an array of pixels, each pixel including a sensing element (e.g., a photodiode) and access circuitry. To facilitate the passage of light to the photodiode, each pixel includes a via wave guide (VWG) defined in the metallization layer formed over the pixel&#39;s photodiode. The VWG includes an upper light concentrator having a curved (e.g., parabolic) surface extending from a relatively wide upper opening to a relatively small lower opening. The VWG also includes a lower section extending between the lower opening of the light concentrator and the associated photodiode. A mirror coating is optionally formed on the surface of the VWG. An optional light-guiding material and/or color filter materials are disposed inside the VWG. An optional microlens is formed over the VWG.

FIELD OF THE INVENTION

The present invention relates to solid state image sensors. Morespecifically, the present invention relates to CMOS image sensors (CISs)having via wave guides, and to methods for making such CISs.

BACKGROUND OF THE INVENTION

Solid state image sensors are used, for example, in video cameras, andare presently realized in a number of forms including charge-coupleddevices (CCDs) and CMOS image sensors (CISs). CISs sensors are based ona two dimensional array of pixels that are fabricated using CMOSfabrication techniques. Each CIS pixel includes a sensing element (e.g.,a photodiode) and access circuitry that are fabricated on asemiconductor substrate, and connected to control circuits by way ofmetal address and signal lines. These metal lines are supported ininsulation material that is deposited over the upper surface of thesemiconductor substrate, and positioned along the peripheral edges ofthe pixels to allow light to pass between the metal lines to the sensingelements through the insulation material. In color image sensors, eachpixel also includes a color filter located over the sensing element. Anarray of microlenses is sometimes located over the metallization layerto focuses light from an optical image through the color filter and theinsulation material into the image sensing elements. Each image sensingelement is capable of converting a portion of the optical image passedby the color filter into an electronic signal. The electronic signalsfrom all of the image sensing elements are then used to regenerate theoptical image on, for example, a video monitor.

The quality of an image generated by a conventional CIS is at least inpart determined by the amount of light that reaches the photodiode ofeach pixel. As indicated above, the photodiode of each pixel covers onlya portion of the entire pixel area, with the access circuitry andaddress/signal lines taking up the remaining CIS surface area.Accordingly, in the absence of microlenses, only a portion of the lightincident on the upper surface of the CIS is captured by the photodiodes.Further, when color filters are present, only a portion of the lightdirected toward a particular photodiode is passed by the color filter,further reducing the amount of captured light that can be used togenerate image information. Moreover, because the light must passthrough the semi-opaque insulation material of the metallization layer,a portion of the filtered light directed toward each photodiode isreflected or refracted away from the photodiode. Some of thisreflected/refracted light may strike an adjacent photodiode, producingblurring and/or inaccurate image color.

What is needed is a CIS that facilitates enhanced image detection byproviding a structure for capturing and concentrating substantially allof the light incident on the CIS, and directing the concentrated lightonto the CIS's photodiodes.

SUMMARY OF THE INVENTION

The present invention is directed to image sensors (e.g., CMOS imagesensors (CISs)) in which each pixel includes a via wave guide defined inthe metallization layer disposed over the pixel's photodiode, where eachvia wave guide includes a relatively large light concentrator formedover the metal lines of the metallization layer, and a relatively narrowlower section extending between the light concentrator and the pixel'sphotodiode through the space separating the metal lines. In accordancewith the present invention, the light concentrator includes a curved(e.g., parabolic) surface shaped such that light beams directed into thelight concentrator are redirected by a suitable light-guiding materiallayer formed on the curved surface into the lower section and toward thephotodiode. By forming via wave guides for each pixel in which the lightconcentrator has an upper opening that is substantially as large as thearea occupied by the associated pixel, the present invention facilitatesenhanced image detection because substantially all of the light directedonto the CIS is concentrated and directed onto the CIS's photodiodes. Inaddition, because the via wave guides facilitate the substantiallytransparent passage for light passing through the metallization layer tothe photodiode, the thickness of the metallization layer is less of anissue than in conventional arrangements, and as such the presentinvention facilitates the production of complex image sensors havingfour or more layers of metal lines over the control circuitry located onthe array periphery.

In accordance with an aspect of the present invention, each via waveguide is filled with a light-guiding material that facilitates passageof light to the pixel's photodiode. In one embodiment, the light-guidingmaterial has a higher refractive index than a refractive index ofinsulation material utilized to form the surrounding metallizationlayer. When disposed in the light concentrator section of the via waveguide, this high refractive index (high-RI) material facilitatesredirecting light beams into the lower section of the via wave guide byrefracting (bending) the light beams in a manner defined by the curvedsurface of the light concentrator.

In accordance with an optional aspect of the present invention, thelight-guiding material comprises a mirror coating disposed over at leastone of the curved surface of the light concentrator and a peripheralsurface of the lower section. The mirror coating located in the lightconcentrator has a curved shape defined by the curved surface of thelight concentrator, thus facilitating the reflection of light beamsentering the light concentrator into the lower section of the via waveguide. The light beams are further reflected by the mirror coatingformed on a peripheral wall of the lower section toward the pixel'sphotodiode. In one embodiment, the mirror coating is formed over apassivation layer. In another embodiment, a transparent light-guidingmaterial is disposed on a surface of the mirror coating.

In accordance with an optional aspect of the present invention, a colorfilter material is inside at least one of the curved surface of thelight concentrator and a peripheral surface of the lower section. Byplacing the color filter material inside the via wave guide, thefiltered light travels a shorter distance to the photodiode, thusreducing the chance of color inaccuracies. In one embodiment, the colormaterial is mixed with a light-guiding material.

In accordance with an optional aspect of the present invention, amicrolens is optionally disposed over the via wave guide to furtherfacilitate the capture and concentration of light directed toward thehost CIS.

In accordance with another embodiment of the present invention, aprocess for forming via wave guides includes dry etching a relativelyshallow hole into an upper insulation layer over a pixel's photodiode,and then applying a wet etch into the hole in a manner that produces alight concentrator having the desired curved (e.g. parabolic) surface. Asubsequent dry etch is then utilized to produce the lower section of thevia wave guide.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings, where:

FIG. 1 is a top side perspective view showing a portion of a CISincluding a pixel having a via wave guide formed in accordance with anembodiment of the present invention;

FIG. 2 is a cross-sectional side view showing a portion of the CIS pixelof FIG. 1;

FIG. 3 is a cross-sectional side view depicting the CIS pixel of FIG. 1during operation;

FIGS. 4(A) and 4(B) are cross-sectional side views showing CIS pixelsincluding via wave guides having high refractive index light-guidingmaterials in accordance with alternative embodiments of the presentinvention;

FIGS. 5(A), 5(B), and 5(C) are cross-sectional side views showing CISpixels including via wave guides having mirror coatings formed inaccordance with additional alternative embodiments of the presentinvention;

FIGS. 6A), 6(B), 6(C) and 6(D) are cross-sectional side views showingCIS pixels including via wave guides having color filter materialsformed in accordance with further additional alternative embodiments ofthe present invention;

FIGS. 7(A), 7(B) and 7(C) are cross-sections showing CIS pixelsincluding via wave guides having microlenses in accordance with furtheradditional alternative embodiments of the present invention;

FIGS. 8(A), 8(B) and 8(C) are cross-sections showing a fabricationprocess for forming the curved light concentrator and the lower sectionof a via wave guide according to another embodiment of the presentinvention;

FIGS. 9(A), 9(B), 9(C), 9(D) and 9(E) are cross-sections showing afabrication process for forming a mirror coating on the curved lightconcentrator and the lower section according to another embodiment ofthe present invention; and

FIGS. 10(A), 10(B) and 10(C) are cross-sections showing a fabricationprocess for forming a microlens over a via wave guide according toanother embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention relates to an improvement in CIS devices involvingan improved via wave guide. The following description is presented toenable one of ordinary skill in the art to make and use the invention asprovided in the context of a particular application and itsrequirements. As used herein, directional terms such as “upper”,“upwards”, “lower”, “downward”, “front”, “rear”, are intended to providerelative positions for purposes of description, and are not intended todesignate an absolute frame of reference. Various modifications to thepreferred embodiment will be apparent to those with skill in the art,and the general principles defined herein may be applied to otherembodiments. Therefore, the present invention is not intended to belimited to the particular embodiments shown and described, but is to beaccorded the widest scope consistent with the principles and novelfeatures herein disclosed.

FIGS. 1 and 2 are perspective and cross-sectional side views showing aportion of a CMOS image sensor (CIS) 100 according to an embodiment ofthe present invention. CIS 100 generally includes a semiconductor (e.g.,monocrystalline silicon) 101, and an array of pixels 110 (one shown) anda metallization layer 120 that are formed on and over substrate 101according to known CMOS fabrication techniques. As indicated in FIG. 1,each pixel 110 includes access circuitry (e.g., an access transistor112) and a photodiode (sensing element) 115 that are formed in apredefined assigned area (indicated by dashed square) on the uppersurface of substrate 101. As indicated in FIG. 2, metallization layer120 includes a series of insulating layers and metal lines that areformed over substrate 101. As defined herein, metallization layer 120includes one or more lower insulation layers 122 that support one ormore metal lines 125, and one or more upper insulation layers 127 thatare formed over the uppermost metal lines 125. For example, as indicatedin FIG. 2, lower insulating layers 122-1, 122-2, and 122-3 arerespectively formed on an upper surface of substrate 101, with a firstlayer of metal lines (including metal line 125-1) supported betweeninsulating layers 122-1 and 122-2, and a third layer of metal lines(including uppermost metal line 125-3) supported on insulating layer122-3.

A via wave guide (VWG) 130 is defined in metallization layer 120 overeach pixel 110, and serves to guide light beams through metallizationlayer 120 to associated photodiode 115. In accordance with the presentinvention, VWG 130 includes both relatively wide light concentratorsection 132 that is defined in upper insulation layers 127 (i.e., aboveuppermost metal lines 125-3), and a relatively narrow lower section 134that is defined in lower insulation layers 122.

As indicated in FIG. 2, light concentrator 132 includes an upper opening136 having a relatively large diameter D1, a lower opening 138 having arelatively small diameter D2, and a curved (e.g., parabolic) surface 139that tapers in a continuous smooth curve between upper opening 136 andlower opening 138. In one embodiment, upper opening 136 is substantiallyequal in size to the area (depicted by the dashed square in FIG. 1)associated with pixel 110. As indicated in FIG. 3, light concentrator132 is shaped such that, when curved surface 139 is coated with asuitable light-guiding (e.g., reflecting or refracting) material, lightbeams LB directed toward pixel 110 are redirected by curved surface 139through the lower opening 138 and into lower section 134. In particular,relatively wide upper opening 136 and curved surface 139 facilitatecapturing a relatively large amount of light directed toward pixel 110,and facilitate redirecting (i.e., by providing a suitable surface anglefor the light-guiding material) the captured light toward lower section134, thereby effectively concentrating the captured light. As discussedin additional detail below, when filled with light-guiding materialshaving a relatively high refractive index (RI), or when coated withmirror materials, the VWG both maximizes the amount of light reachingassociated photodetector 115, and minimizes cross-talk with neighboringpixels (not shown). In addition as depicted in FIG. 3 bydashed-dot-lined arrow LB-A, another benefit of the present invention isthat curved surface 139 enables the capture and concentration of a widerange of incident light angles without the use of microlenses.Accordingly, VWG 130 facilitates enhanced image detection becausesubstantially all of the light directed onto CIS 100 is concentrated anddirected onto the CIS's photodiodes (e.g., photodiode 115).

Referring to FIG. 2, in accordance with an embodiment of the presentinvention, lower section 134 of VWG 130 is substantially verticallyaligned in lower insulating section 122 of metallization layer 120, andextends between lower opening 134 of light concentrator 132 andphotodiode 115. A peripheral surface 135 of lower section 134, which isdefined by the surrounding insulation material, defines one of asubstantially square cross-section, a substantially circularcross-section, and a substantially octagonal cross-section, depending onthe fabrication process technique utilized to etch the insulationmaterial.

FIGS. 4(A) and 4(B) are cross-sectional side views showing portions of aCIS 100-1A and a CIS 100-1B that include pixels 110-1A and 110-1B,respectively, which in turn include VWG 130-1A and 130-1B, respectively.VWG 130-1A and VWG 130-1B differ from VWG 130 (described above) in thatthey include a mirror coating 150 disposed on at least one of curvedsurface 139 of light concentrator 132 and peripheral surface 135 of VWGlower section 134, and has a high refractive index (high-RI)light-guiding material 140 disposed in their respective lightconcentrators, which are formed in the manner described above to includecurved surface 139. As defined herein, high-RI light-guiding material140 has a higher refractive index than the refractive index ofinsulation material 121 forming the various layers of metallizationlayer 120. In an exemplary embodiment, high-RI light-guiding material140 includes at least one of silicon-nitride (SiN) and titanium-oxide(TiO2) based polymers. Referring to FIG. 4(A), in one embodiment, VWG130-1A includes high-RI material 140 disposed in both light concentrator132 and in lower section 134, and mirror coating 150 is disposed only onperipheral surface 135 of VWG lower section 134. In the alternativeembodiment shown in FIG. 4(B), VWG 130-1B includes both high-RI material140 and mirror coating 150 disposed in light concentrator 132 and lowersection 134. VWG 130-1B also includes an optional anti-reflectivecoating (layer) 142 (e.g., silicon-on-glass (SOG) or any other materialwith a lower refractive index than that of the high-RI material) formedon upper surface 141 and upper surface 129 of metallization layer 120.Anti-reflective coating 142 is particularly useful when mirror coating150 is a relatively low reflectance material (e.g., tantalum ortitanium, versus a relatively highly reflective material such asaluminum). In this case, high-RI material 140 produces only onereflection (or a minimum number of reflections) from mirror coating 150,thereby reducing the light loss when the light hits mirror coating 150.In this instance, anti-reflective coating 142 serves to minimize thereflectance losses from the transition between air and hi-RI layers. Theembodiment illustrated in FIG. 4(B) may be further modified to includethe color filter material (not shown) in the manner described below, ordisposed over anti-reflective coating 142. In another alternativeembodiment (not shown), lower section 134 is filled with a transparentlight-guiding material 145 having a refractive index that is relativelylow in comparison to that of high-RI material 140. Suitable transparentmaterials 145 include, for example, silicon-dioxide (SiO₂) and spin-onglass, which is typically used only if lower section 134 is covered witha mirror.

FIG. 5(A) is a cross-sectional side view showing a portion of a CIS100-2A that includes a pixel 110-2A, which in turn includes a VWG 130-2Athat is formed in accordance with another embodiment of the presentinvention. VWG 130-2A differs from VWG 130 (described above) in that VWG130-2A includes a mirror coating 150 disposed on at least one of curvedsurface 139 of light concentrator 132 and peripheral surface 135 of VWGlower section 134. As defined herein, mirror coating 150 ischaracterized as being substantially fully reflective to light beamsentering through upper opening 136. In an exemplary embodiment, mirrorcoating 150 includes at least one of aluminum, tantalum, tungsten,titanium, silver, gold, platinum, and copper. When formed in lightconcentrator 132, an outer surface 151 of mirror coating 150 issubstantially coincident with and shaped by curved surface 139 to form,for example, a parabolic mirror structure that reflects light enteringthrough upper opening 136 into lower section 134, thereby facilitatingefficient concentration and transmission of light entering ontophotodiode 115. When light-reflective material is disposed on thesurfaces of both light concentrator 132 and lower section 134, as shownin FIG. 5(A), mirror coating 150 effectively forms light-capturing andconcentrating mirror tunnel that directs substantially all of the lightbeams directed toward upper surface 129 over pixel 110-1A to itsphotodiode 115. Further, the lower portion of mirror coating 150substantially shields photodiode 115 from receiving “stray” light beams(e.g., light beam LB5A) that enter metallization layer 120 outside ofmirror coating 150, whereby cross talk between adjacent pixels can beentirely eliminated. VWG 130-2A also includes and optional transparentlight-guiding material 145 (e.g., an amorphous polymer or a dielectricmaterial) that is disposed on an inside surface of mirror coating 150 inat least one of lower section 134 and light concentrator 132. Thepresence of light-guiding material 145 provides protection forphotodiode 115 and a stable base for structures formed overmetallization layer 120, and further serves to enhance lightconcentration. In an alternative embodiment, the area inside mirrorcoating 150 may remain empty (i.e., air filled).

FIG. 5(B) is a cross-sectional side view showing a portion of a CIS100-2B that includes a pixel 110-2B, which in turn includes a VWG 130-2Bthat is formed in accordance with yet another embodiment of the presentinvention. VWG 130-2B differs from VWG 130-2A in that VWG 130-2Bincludes a passivation layer 155 that is disposed between metallizationlayer 120 and mirror coating 150. Passivation layer 155 includes, forexample, silicon nitride and silicon dioxide, and serves to provide asmooth surface for mirror coating 150, and to provide electricalinsulation between mirror coating 150 and the metal lines 125 when metallines 125 are unintentionally exposed during the VWG etch process.

FIG. 5(C) is a cross-sectional side view showing a portion of a CIS100-2C that includes a pixel 110-2C, which in turn includes a VWG 130-2Cthat is formed in accordance with yet another embodiment of the presentinvention. VWG 130-2C includes mirror coating 150 and optionalpassivation layer 155, described above. However, mirror coating 150 isdisposed only on curved surface 139 of the light concentrator 132 (i.e.,not on peripheral wall 135 of lower section 134), and high-RIlight-guiding material 140 (described above) is disposed in lowersection 134. In addition, VWG 130-2C includes an optional transparentlight-guiding material 145 (e.g., an amorphous polymer or a dielectricmaterial) that is disposed on an inside surface of mirror coating 150 inlight concentrator 132.

FIGS. 6(A) to 6(D) are cross-sectional side views showing portions ofCIS 100-3A to 100-3D that include pixels 110-3A to 110-3D, respectively,which in turn include VWGs 130-3A and 130-3D, respectively. Each VWG130-3A to 130-3D includes a light concentrator 132 and a lower section134 that are substantially as described above. However, VWGs 130-3A to130-3D differ from previous embodiments in that they include a colorfilter material 160 disposed in at least one of light concentrator 132and lower section 134. The benefit of disposing color filter material160 inside VWGs 130-3A to 130-3D is that this arrangement facilitatescolor filtering in close proximity to the associated photodiode 115,thereby avoiding cross-talk in the form of light passed by adjacentcolor filters from generating inaccurate detection by associated colorfilter 115. Note, however, that the thickness T_(CFM) of color filtermaterial 160 is preferably substantially equal to the thickness of colorfilters in conventional arrangements, unless the color filter materialis mixed/diluted (as described below with reference to FIG. 6(D)).

FIG. 6(A) depicts a VWG 130-3A formed in accordance with a firstexemplary embodiment, where VWG 130-3A includes a high-RI light-guidingmaterial 140 disposed in lower section 134, and color filter material160 is deposited over a mirror coating 150, which is formed in themanner described above, where both mirror coating 150 and color filtermaterial 160 are disposed in light concentrator 132. In thisarrangement, high-RI light-guiding material 140 serves to support colorfilter 160, thus simplifying the color filter formation process. In oneembodiment, color filter material 160 is either formed from or mixedwith a high refractive index material to facilitate concentration andtransmission of light into lower section 134. As mentioned above, theheight of light concentrator 132 is selected to equal the conventionalcolor filter thickness T_(CFM). In another alternative embodiment, a SOGtopcoat (not shown) is formed over VWG 130-3A to protect the exposed CFAmaterial from damage and/or contamination. The optional SOG topcoat mayalso be used to open the pads after the formation of the VWG.

FIG. 6(B) depicts a VWG 130-3B formed in accordance with a secondexemplary embodiment, where VWG 130-3B includes color filter material160 disposed in lower section 134 such that a distance between colorfilter material 160 and photodiode 115 is minimized. In one embodiment,color filter material 160 is deposited in lower section 134 and thenetched to provide the required thickness T_(CFM). VWG 130-3B alsoincludes mirror coating 150 disposed on curved wall 139 and along lowersection 134 between light concentrator 132 and color filter material160. With this arrangement, substantially all light entering upperopening 136 is reflected by “full-length” mirror coating 150 throughcolor filter material 160 onto photodiode 115, thereby completelyeliminating cross-talk between adjacent color filtered pixels (e.g.,green filtered light will only reach the photodiode located under thegreen filter material, and this photodiode will be shielded by themirror coating from receiving light from red or blue filters, othergreen filters, or stray “white” light). As in previous embodiments, atransparent light-guiding material (not shown) may be optionally used tofill the otherwise empty space inside light concentrator 132 an in lowersection 134 between above color filter material 160.

FIG. 6(C) depicts a VWG 130-3C formed in accordance with a thirdexemplary embodiment, where, similar to VWG 130-3B, VWG 130-3C includesa filtering material 160 disposed in lower section 134 in a way thatminimizes the distance between color filter material 160 and photodiode115. VWG 130-3C also includes high-RI light-guiding material 140disposed on curved wall 139 and along lower section 134 between lightconcentrator 132 and color filter material 160. With this arrangement,most of the light entering upper opening 136 is refracted through colorfilter material 160 onto photodiode 115.

FIG. 6(D) depicts a VWG 130-3D formed in accordance with a thirdexemplary embodiment, where VWG 130-3D includes a color filter mixture165 that is formed by dispersing (mixing or otherwise diluting) thecolor filter material (discussed above) in one of the light-guidingmaterials described above. Mixing the color filter material with thelight-guiding material provides a benefit of eliminating the need forcontrolling the thickness of the color filter material. That is, asdiscussed above, when the color filter material is unmixed as shown inFIGS. 6(A) and 6(B), the thickness T_(CFM) of the resulting color filterstructure 160 must be etched or otherwise controlled to achieve thedesired color filtering characteristic. By mixing the color filtermaterial in an appropriate amount of one of the low RI transparentmaterials described above, the desired color filtering characteristicmay be achieved without the need for performing a separate color filteretch. Note that the amount of transparent material (i.e., the level ofdilution) is determined, e.g., by the overall height H of VWG 130-3D.Finally, mirror coating 150 is used in the manner described above tofacilitate transmission of light to photodiode 115.

FIGS. 7(A) to 7(C) are cross-sectional side views showing portions ofCIS 100-4A to 100-4C that include pixels 110-4A to 110-4C, respectively,which in turn include VWGs 130-4A and 130-4C, respectively. Each VWG130-4A to 130-4C includes a light concentrator 132 and a lower section134 that are substantially as described above. VWGs 130-4A and 130-4Bdiffer from previous embodiments in that they include a microlens 170disposed over upper opening 136 of light concentrator 132. As mentionedabove, one advantage of the present invention is that the various VWGsreduce or eliminate the need for microlenses. However, in someapplications the use of microlenses in conjunction with the VWGs of thepresent invention may provide superior performance.

In accordance with an aspect of the present invention, VWGs 130-4A and130-4B are at least partially filled with a material capable ofsupporting microlenses 170. As indicated in the exemplary embodimentdisclosed in FIG. 7(A), VWG 130-4A includes mirror coating 150 formed oncurved surface 139 and along lower section 134. In addition, disposedinside mirror coating 150 are one or more of light guiding material 145,color filter material 160 and transparent/color filter mixture 165,which support microlens 170. In the alternative exemplary embodimentdisclosed in FIG. 7(B), VWG 130-4B includes high-RI light-guidingmaterial 140 disposed inside light concentrator 132 and color filtermaterial 160 disposed in lower section 134, with microlens 170 disposedon light-guiding material 140. In an alternative embodiment (not shown),high-RI material is disposed in the lower section and color filtermaterial is disposed in the upper section (in the tapered lightconcentrator), with a microlens disposed above the color filtermaterial.

FIG. 7(C) shows another alternative embodiment of the present inventionin which a VWG 130-4C includes a microlens 175 disposed inside lowersection 134 directly over photodiode 115. Microlens 175 is formed, forexample, by depositing resist inside lower section 134, and melting thephotoresist using known techniques to produce a suitable lens structure.In one embodiment, microlens 175 is formed after the formation of mirrorcoating 150, which is depicted as being formed on passivation layer 155.Subsequent to the formation of microlens 175, one or more of transparentlight-guiding material 145 and color filter material 160 may be formedin VWG 130-4C in the manner described above. As indicated by the dashedline structure, in another optional embodiment, a “big” microlens 170 isadded above VWG 130-4C as in the previous embodiments to further focuslight.

FIGS. 8(A) to 8(C) are cross-sectional side views illustrating a processfor fabricating via wave guides according to another embodiment of thepresent invention.

Referring to FIG. 8(A), standard CMOS processes may be used to fabricatephotodiode 115 and access circuitry (not shown) in substrate 101.Subsequently, metallization layer 120 is formed over substrate 101 usingstandard CMOS techniques such that metallization layer 120 includeslower insulation layers 122 and several layers of metal lines 125-1 to125-3 respectively disposed between insulation layers 122-1 to 122-3 inthe manner described above with reference to FIG. 1. After forminguppermost metal lines 125-3, one or more upper insulation layers 127 areformed according to standard CMOS fabrication techniques. In oneembodiment, upper insulation layers 127 comprise silicon dioxide thatmay be covered by silicon-nitride.

In a first stage of the via wave guide formation process, a first mask802 is formed over an upper surface of upper insulation layers 127, anda window (mask opening) 805 is patterned into mask 802 such that window805 exposes an upper surface of upper insulation layers 127 and islocated over photodiode 115. Next, a dry etching process is performed byapplying a directional dry etchant 810 through window 805 that forms aprecursor hole 812 in upper insulation layers 127. The size (i.e., widthand depth) of precursor hole 812 is selected to facilitate the formationof the desired light concentrator, as described below. In oneembodiment, precursor hole 812 a first, relatively short distancebetween the upper surface and the photodiode (relative to the finaldepth of the subsequently completed via wave guide).

Referring to FIG. 8(B), a wet etchant 820 is then applied over firstmask 802 such that wet etchant 820 enters window 805 and etches upperinsulation layer 127. By selectively matching the size of precursor hole812 and the etching rate and time of wet etchant 820, the wet etchingprocess is controlled to produce light concentrator 132 having curvedwalls 139 forming a substantially parabolic shape having a relativelywide upper opening 134 located adjacent to first mask 802, and taperingto a relatively narrow lower end 138A.

As shown in FIG. 8(C), following the wet etch used to form lightconcentrator 132, dry etchant 810 is again applied through mask opening805 to define lower opening 138 of light concentrator 132, and to formlower section 134 in lower insulation layers 122. Note that lowersection 134 extends substantially vertically between light concentrator132 and photodiode 115, but may not extend all the way to photodiode 115in the manner depicted (i.e., the etching process may be terminatedbefore etching entirely through lower insulation layers 122 to preventdamage to photodiode 115). Note that, depending on the shape of window805 and the applied power utilized during the dry etching process, lowersection 134 is formed with a substantially uniform (e.g., substantiallysquare, circular, or octagonal) cross-section.

Upon completing the dry etching process used to form lower section 134that is described above with reference to FIG. 8(C), basic VWG 130 isdefined in metallization layer 120 that may be further processed to formany of the various embodiments described above.

FIGS. 9(A) to 9(E) illustrate the formation of a mirror coating oncurved surface 139 and peripheral wall 135 of VWG 130 according to anexemplary embodiment of the present invention. Referring to FIG. 9(A), athin passivation layer 155 (e.g., SiO₂ on a thin layer of SiN) isdeposited on curved surface 139 and peripheral wall 135 using standardtechniques. Note that a lower portion 905 of passivation layer 155 isformed over photodiode 115. Next, as shown in FIG. 9(B), a lightreflective material layer 910 is formed over passivation layer 155. Inone embodiment, formation of light reflective material layer 910involves depositing at least one metal selected from the group includingaluminum, tantalum, tungsten, titanium, silver, gold, platinum, andcopper by, for example, sputtering, chemical vapor deposition (CVD)(e.g., conformal coating such as aluminum CVD), evaporation, orre-sputter techniques (e.g., tantalum deposition and re-sputter). Notethat a lower end portion 915 of light reflective material layer 910 isformed on lower portion 905 of passivation layer 155. FIG. 9(C)illustrates the subsequent step of forming a protective (masking) layer920 (e.g., SiO₂) over light reflective material layer 910 using standarddeposition techniques. As indicated in FIG. 9(D), a directional dry etch930 is then utilized to remove the portions of protective layer 920 thatare formed on horizontal surfaces, including the small portion ofmasking layer 920 formed over lower end portion 915 of light reflectivematerial layer 910. Note that a portion of protective layer 920 remainsattached to curved surface 139 of light collector 134, and that theselectivity of dry etch 930 may be set such that lower end portion 915is etched faster than protective layer 920 after removing the protectivematerial located over lower end portion 915. As shown in FIG. 9(E), ametal etchant 940, which is determined by the type of light reflectivematerial utilized to form layer 910, is applied to remove the exposedportions of the light reflective material layer 910, thereby completingthe formation of mirror coating 150 over curved surface 139 of lightconcentrator 132 and peripheral surface 135 of lower section 134.Although not indicated in subsequent figures, masking layer 920 ispreferably left on mirror coating 150 following the metal etch. Also, inone embodiment, metal layer portions 920-1 formed over upper surface 129(shown in dashed lines in FIG. 9(E)) are retained to prevent light fromentering metallization layer 120 and potentially generating cross talk.Note that the above process for removing lower end portion 915 isexemplary, and those skilled in the art will recognize this removalprocess may be achieved using other known approaches.

FIGS. 10(A) to 10(C) illustrate a process for forming a microlens over avia wave guide in accordance with another embodiment of the presentinvention. The exemplary embodiment shown in FIGS. 10(A) to 10(C)includes a mirror coating 150 inside light concentrator 132 and lowersection 134. First, a support structure, comprising at least one oftransparent light-guiding material 145, color filter material 160, ormixed color filter material 165 (described above), is disposed insidelight concentrator 132 and lower section 134 in order to support thesubsequently formed microlens. As shown in FIG. 10(A), an optionalsecond mask 1010 is formed on upper surface 129 of metallization layer120, and the selected support materials are deposited through a window1015 into light concentrator 132 and lower section 134 using knowntechniques. In one embodiment, when light-guiding material 145 is used,the material is inserted into the VWG by spin coating without using mask1010. Alternatively, if a photoresist is used to fill the VWG, mask 1010may be used as shown. When color filter material or a mixture is used,then deposition by spin coating and then exposing each color using anassociated mask (i.e., three masks for the three different colors). Asindicated in FIG. 10(B), mask 1010 is then removed, and a planarizingprocess (e.g., CMP, etch back or coating with another planarizing layer)is performed using a suitable etchant 1020 such that the upper surfaceof material 145/160/165 located at upper opening 136 is coplanar withupper surface 129 of metallization layer 120. FIG. 10(C) illustrates thesubsequent step of forming microlens 170 over planarized material145/160/165 using known microlens forming techniques. Note that the useof microlens 170 may reduce the need for mirror coating 150, and mayprovide a suitable VWG structure in combination with high-RI lightguiding material 140 alone (e.g., similar to VWG 130-4B, shown in FIG.7(B)).

Although the present invention has been described with respect tocertain specific embodiments, it will be clear to those skilled in theart that the inventive features of the present invention are applicableto other embodiments as well, all of which are intended to fall withinthe scope of the present invention. For example, although the presentinvention is described with specific reference to CIS devices, thepresent invention may be utilized to generate other types of imagesensors as well. Moreover, although the ideal size of upper VWG opening136 is substantially equal to the pixel size, the inventors believe itmay in some circumstances be necessarily smaller (e.g., by 0.2 to 0.6microns) than the pixel size due to process fabrication problems (e.g.,a large etch bias can result in walls being etched completely through).Further, the dry-wet-dry etching process described above with referenceto FIGS. 8(A) to 8(C) for forming parabolic light concentrators may bereplaced with a dual damascene-like process.

1. An image sensor (CIS) comprising: a sensing element formed in asubstrate; and a metallization layer formed over the substrate, whereinthe metallization layer defines a via wave guide extending through aportion of the metallization layer located over the sensing element, thevia wave guide including a light concentrator having a relatively largeupper opening, a relatively small lower opening, and a curved surfaceextending between the upper and lower openings, and wherein the curvedsurface is shaped such that light beams directed toward the curvedsurface are redirected through the lower opening of the via wave guidetoward the sensing element.
 2. The CIS of claim 1, wherein the curvedsurface is a parabolic surface.
 3. The CIS of claim 1, wherein the viawave guide further comprises a lower section having a peripheral surfacedefined in the metallization layer and extending between the loweropening of the light concentrator and the sensing element.
 4. The CIS ofclaim 3, wherein the peripheral surface of the lower section comprisesone of a substantially square cross-section, a substantially circularcross-section, and a substantially octagonal cross-section.
 5. The CISof claim 3, wherein the CIS further comprises plurality of pixelsarranged in an array, each of the plurality of pixels including anassociated sensing element and one or more components occupying anassociated area of the substrate and, wherein the associated sensingelement is coupled between the associated sensing element and at leastone metal wire disposed in the metallization layer, and wherein theupper opening of the light concentrator associated with each pixel issubstantially equal in size to the area of said associated each pixel.6. The CIS of claim 3, wherein the CIS further comprises a light-guidingmaterial disposed in the via wave guide.
 7. The CIS of claim 6, whereinthe metallization layer comprises a plurality of metal wires disposed inan insulation material, and wherein the light-guiding material has ahigher refractive index than a refractive index of the insulationmaterial of the metallization layer.
 8. The CIS of claim 7, wherein thelight-guiding material comprises at least one of SiN and TiO₂ basedpolymers.
 9. The CIS of claim 6, wherein the light-guiding materialcomprises at least one of an amorphous polymer, SiO₂ and glass.
 10. TheCIS of claim 3, further comprising a mirror coating disposed over atleast one of the curved surface of the light concentrator and theperipheral surface of the lower section.
 11. The CIS of claim 10,wherein the mirror coating comprises at least one of aluminum, tantalum,tungsten, titanium, silver, gold, platinum, and copper.
 12. The CIS ofclaim 10, further comprising a light transparent material disposed on aninside surface of the mirror coating.
 13. The CIS of claim 10, furthercomprising a passivation layer disposed between the mirror coating andat least one of said curved surface of the light concentrator and saidperipheral surface of the lower section.
 14. The CIS of claim 10,wherein the mirror coating is disposed on the curved surface of thelight concentrator, wherein the metallization layer comprises aplurality of metal wires disposed in an insulation material, wherein theCIS further comprises a light-guiding material disposed in the lowersection of the via wave guide, and wherein the light-guiding materialhas a higher refractive index than a refractive index of the insulationmaterial of the metallization layer.
 15. The CIS of claim 3, furthercomprising a color filter material disposed in the via wave guide. 16.The CIS of claim 15, wherein the color filter material is disposed inthe light concentrator, and at least one of a transparent material and amaterial having a relatively high refractive index is disposed in thelower section of the via wave guide.
 17. The CIS of claim 15, whereinthe color filter material is disposed in the lower section of the viawave guide, and wherein one of a mirror coating and a material having arelatively high refractive index is disposed in the light concentrator.18. The CIS of claim 15, wherein the color filter material is dispersedin a transparent material.
 19. The CIS of claim 1, further comprising amicrolens disposed over the light concentrator of the via wave guide.20. The CIS of claim 3, further comprising a microlens disposed in thelower section of the via wave guide.
 21. The CIS of claim 20, furthercomprising a second microlens disposed over the light concentrator ofthe via wave guide.
 22. A method for fabricating a via wave guide in aCMOS image sensor (CIS), the method comprising: forming a sensingelement in a substrate; forming a metallization layer over the sensingelement, wherein the metallization layer includes a plurality ofinsulation layers and a plurality of metal lines disposed in theinsulation layers, and having an upper surface; dry etching themetallization layer through a mask opening to define a first, relativelynarrow hole through the upper surface of the metallization layer, thefirst hole extending a first, relatively short distance between theupper surface and the sensing element; and wet etching the metallizationlayer through the mask opening to define an light concentrator of thevia wave guide, the light concentrator having a first, relatively wideopening located adjacent to the upper surface and a tapered surfaceextending between the upper opening and a lower end.
 23. The methodaccording to claim 22, wherein defining the light concentrator comprisesforming a parabolic surface.
 24. The method of claim 22, furthercomprising dry etching the metallization layer through the mask openingto define an lower section of the via wave guide such that a peripheralsurface of the lower section has a substantially uniform cross sectionextending from the lower end of the light concentrator toward thesensing element.
 25. The method according to claim 24, furthercomprising forming a mirror coating on the curved surface of the lightconcentrator.
 26. The method according to claim 25, wherein forming themirror coating comprises: depositing a passivation layer on the curvedsurface of the light concentrator; forming a light reflective materiallayer on the passivation layer; and removing a portion of the lightreflective material layer located at a lower end of the via wave guide.27. The method according to claim 26, wherein removing the portion ofthe light reflective material layer located at a lower end of the viawave guide comprises: forming a protective layer layer over the lightreflective material layer; dry etching the protective layer such thatthe portion of the light reflective material is exposed and such that aremaining portion of the protective layer remains attached to the curvedsurface of the light collector; and etching the exposed portion of thelight reflective material layer such that the remaining portion of thepassivation layer protects the light reflective material layer formed onthe curved surface of the light collector.
 28. The method according toclaim 22, further comprising disposing at least one of a color filtermaterial and a light-guiding material in the via wave guide.
 29. Themethod according to claim 22, further comprising forming a microlensover the light concentrator of the via wave guide.